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Among seabirds to polar bears, primates to orangutans, certain species engage in mouth-to-mouth contact. Now, scientists propose that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were intimately acquainted. In previous studies, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the concept chimed with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

Publishing in the publication a scientific periodical, Brindle and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not restricted by how people kiss.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's very much been focused on humans, which means that essentially non-human species do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

As a result the research group came up with a definition of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Study Approach

Brindle said they focused on accounts of kissing in primates from the African continent and Asia, including primates, apes and great apes, and employed online videos to confirm the observations.

Scientists then combined this data with details on the genetic connections between living and ancient species of such animals.

Historical Origins

Researchers propose the findings suggest intimate contact evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the researchers conclude. But the activity might not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we currently have demonstrated that ancient relatives very likely kissed, suggests that the two [species] are also likely to have engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is discussed, the expert said intimate contact could be used in sexual contexts to potentially increase mating outcomes or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates said that as kissing behavior was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of various types of intimate behavior among a broader range of animals might extend its beginnings back further still.

"Things that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Social Elements

An archaeology expert said that kissing had a social component as it was not common to all societies.

"However, as people we thrive or fail on the quality of our emotional bonds, and methods of promoting confidence and intimacy will have been significant for eons," the professor stated. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be expected that ancient hominins – and including them and our own species collectively – kissed."